1. Field of the Invention
Embodiments of the invention relate generally to optical systems, methods, and applications. More particularly, embodiments of the invention are directed to a tunable, achromatizing optical system and method, and applications for use in ophthalmic imaging and other optical systems.
2. Technical Background
Adaptive optics based research instruments and the pervasive use of imaging techniques such as optical coherence tomography has made high-resolution ophthalmic imaging of retinal structure at a cellular scale more the clinical norm rather than the exception. Technological advances such as better deformable mirrors, more efficient light sources and detection channels, robust electronic devices, and software engines for controlling equipment in real-time and for post-processing have brought high-resolution retinal imaging into the era of functional imaging. Some of the most attractive and cutting-edge methodologies currently being developed, include AO-enabled optical coherence tomography, hyper-spectral imaging, and two-photon fluorescence imaging all have one thing in common: all of these modalities use broad-band light sources. The spectral bandwidth of these sources is in many ways essential to the signal efficiency, resolution, and the clinical efficacy of these instruments. With their development, light sources broader than many tens of nanometers to several hundreds of nanometers in bandwidths will soon be used extensively for high-resolution retinal imaging.
Use of such broadband light sources brings with it certain complexities, notably the inherent chromatic aberration properties of the eye. Longitudinal chromatic aberration (LCA) has been measured and documented over a wavelength regime from 450 to 900 nm. The data for human LCA in the visible regime and in the near infrared regime together inform that the average LCA between 450 and 900 nm is about 1.6 D. In other words, the shortest and longest wavelengths in this band end up focusing roughly 300 nm apart in the retina. While this is the average LCA among the population of people recruited for experiments, the LCA varies from person to person and eye to eye; this variation believed by some to be due to differences in corneal radius.
In vivo two-photon imaging has been demonstrated in the living mouse eye. Ultrashort broadband pulses generated in mode-locked lasers are typically used for two-photon excitation. In terms of diopters (D), the mouse eye suffers from substantial longitudinal chromatic aberration (LCA), which is on average almost an order of magnitude greater than that for the human eye. Depending on the bandwidth of the pulsed laser source, this larger amount of LCA may reduce the efficiency of two-photon excitation by as much as 70% or more.
Achromatizing lenses have previously been designed for the human eye in the visible and infrared regimes to correct for the typical range of the LCA in a given eye, demonstrating improvements in ultrahigh resolution optical coherence tomography. Since the LCA of the eye causes different wavelengths to focus at different retinal layers, an achromatizing lens provides equal and opposite wavefront curvature or dioptric power to all wavelengths across the spectral bandwidth of interest (see e.g., FIG. 1). When the light is focused by the eye, all wavelengths converge to the same layer in the retina. Thus, in ophthalmic systems, an achromatizing lens is typically placed in a conjugate pupil plane of the eye and the magnitude of wavefront curvature would depend on the magnification between the ocular pupil plane and the achromatizing lens. For an AOSLO instrument, for example, the entrance pupil (EP) of the optical system or a plane that is conjugate to the EP, is the most suitable location to place such a lens.
A static achromatizing lens functions as it is designed to for one particular eye whose chromatic aberration profile is equal and opposite to that of the lens. However, given the variability of LCA from eye to eye, the use of a static achromatizing lens provides varying levels of uncorrected LCA in different eyes. For broadband imaging modalities, uncorrected LCA can result in different wavelengths being focused too far away from each other. For optical coherence tomography, this will adversely affect the axial resolution of the instrument. For hyperspectral imaging, which relies on capturing the contrast of the tissue at different wavelengths, the lack of reliability of imaging the same layer at different wavelengths is detrimental to the clinical information this modality can convey. For two-photon imaging, depending on the pulse-width of light used the uncorrected LCA can result in spatio-temporal focusing discrepancies that have the potential to decimate the efficiency of two photon fluorescence as well as spatial resolution.
In view of the aforementioned shortcomings known in the art and the challenges presented by uncorrected LCA, it would be advantageous to have a tunable LCA-correcting optical system operable over a spectral range from about 400 nm to about 1000 nm.